Notes

A black hole is a region of spacetime where gravity is so strong that nothing, even particles or even electromagnetic radiation such as light can escape from it.The compact mass can deform spacetime to form a black hole. The boundary of "no escape" is called the event horizon. Although it has an enormous effect on the object crossing it, it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field in curved spacetime predicts that event horizons emit Hawking radiation, with the same style as a black body of a temperature inversely proportional to the mass. Unfortunately, this temperature is on the order of billionths of a "kelvin" for black holes of stellar mass, making it impossible to observe directly.Black holes of stellar mass form when massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow again by absorbing mass from their surroundings. Supermassive black holes of millions of solar masses, may form by absorbing other stars and merging with other black holes. There is supermassive black holes exist in the center of most galaxies.
The Milky Way[a] is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, (galaktikos kyklos), meaning "milky circle. From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

The Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000–200,000 light-years. Recent simulations suggest that a dark matter disk, also containing some visible stars, may extend up to a diameter of almost 2 million light-years.[12][13] The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.[25][26]